Experiment No-1 · The unsaturated values of Xd and Xq for a 3-phase synchronous machine may be...

Prepared By- Ms. Nida Aziz Approved By- Mr. Rohit Kumar

Experiment No-1 Objective: To determine direct axis reactance (xd) and quadrature axis reactance (xq) of a salient pole alternator.

Theory & Concepts:

The unsaturated values of Xd and Xq for a 3-phase synchronous machine may be

found by applying low values of balanced voltage to its armature, and driving its

rotor mechanically at a speed differing slightly from the normal synchronous speed,

the field circuit being open.

The rotating armature m.m.f axis gradually changes, on account of the 'slip '

between coincidenc4 with the polar & interpolar axes successively.

The reluctance of the magnetic circuit varies cyclically between an upper & a lower

limit, and the armature current consequently changes in the reverse sense.

The ratios of applied voltage to armature current gives the synchronous reactances,

using minimum ratio for Xq and maximum for Xd. Xd has the same value as would

be obtained from the normal no load and short circuit tests.

Apparatus: 1. Two A.C voltmeters 2. One A.C ammeter 3. Rheostats 4. A single throw triple pole switch

Dev Bhoomi Institute Of Technology Department of electrical and electronics engg.

LABORATORY MANUAL

PRACTICAL INSTRUCTION SHEET

EXPERIMENT NO. 1 ISSUE NO. : ISSUE DATE:

REV. NO. : REV. DATE :

PAGE: 1

LABORATORY Name & Code :PEE751 POWER SYSTEM LAB SEMESTER:VI


.Procedure: 1. A coupled D.C motor very near to synchronous speed runs the salient- pole synchronous machine. If the synchronous speed is 1500 rpm, the set is run at 1750 rpm. 2. The stator of the salient pole alternator is supplied from a low voltage (10-20 Volts), 3 -phase supply. The supply frequency is more than 50 Hz as the supplying alternator is run at 1750 rpm. 3. The field is kept open and the maximum and minimum deflections in the ammeter (to read the supply voltage) are read. 4. Xd, Xq are calculated. Results: Xd = Maximum V/Minimum I Xq = Minimum V/ Maximum I tests

Discussion: 1. The alternator should not be run at exactly the synchronous speed, for then

instruments will give steady deflections.

2. The stator voltage must be capable of close adjustment.

3. The slip must be very small. Otherwise the measurements will be in error on account

of eddy currents in the pole faces or the damper windings.

4. If in the experiment, the slip is very large, and cannot be adjusted to a smaller value,

he measurements are liable to error.

5. The slip tends to pulsate because of fluctuation of torque with the relative pole

positions with the result that there is a tendency for Xd to be underestimated.


Experiment No-2

Objective:- To determine negative and zero sequence reactances of an alternator. Theory & Concepts: The sequence impedances of an alternator have different values. This is because of the difference in the effect of the armature m.m.f on the DC field m.m.f for different sequences. They may be defined as: Positive Sequence Impedance It is the ratio of the fundamental component of armature voltage, due to the fundamental positive sequence component of armature current, to this component of armature current at rated frequency. This is the usual impedance (either synchronous or transient or subtransient) of alternator. Negative Sequence Impedance It is the ratio of fundamental component of armature voltage, due to the fundamental negative sequence component of armature current, to this component of armature current at rated frequency. Zero Sequence Impedance It is the ratio of fundamental component of armature voltage, due to the fundamental zero sequence component of armature current, to this component of armature current at rated frequency. Procedure: Positive Sequence Impedance (Xd" and Xq")


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FIG.1. Apply a single-phase voltage to two phases (of the stator) in series of a

stationary star-connected alternator. Adjust the rotor position so that the current

due to the induced voltage in the short-circuited field winding is maximum.

Neglecting the resistance , Xd" will be : Xd" = V/2I" where I" is the current flowing

through the two phase of the alternator with an applied voltage V.

Turn the rotor through half pole pitch, q-axis will coincide with crest of the

armature m.m.f and the reactance offered by the armature would now be Xq".

For any rotor position, apply single-phase voltage across two windings in series

(say, phases A and B) and obtain the reactance V/2I".

For the same rotor position, repeat the test for phase B and C and then C and A in

series.

X1,X2 and X3 are he reactances obtained (X1 being maximum). Then, Xd" = K-m Xq"= K+m where K= (X1+X2+X3)/3 m =[ {(X1-K)2 + (X2-X3) 2}/3]1/2 Negative Sequence Impedance (X2) One possible method of measuring X2 is to run he machine at rated speed with its field winding closed and to impress a balanced 3-phase voltage on the armature terminals. Adjust the phase sequence of the voltage so that the current due to this voltage in 3-phase armature windings sets up a rotating field in a direction opposite to the direction of rotation of the machine. The negative sequence impedance is then the ratio of impressed voltage and current per phase. A wattmeter connected in the circuit will give the negative sequence resistance power loss and thus the value of r2. The value of X2 can then be calculated. FIG.2 gives the connection diagram for another method. Two phase A and B of the star-connected alternator are short-circuited and voltage V is measured between C and the junction terminals A and B. The machine is driven at rated speed. The negative sequence impedance is then Z2 = V/(Ö3I) The value of X2 can be obtained with the help of the wattmeter reading P,


as X2 =PZ2/(VI) Short-circuit current is kept low so that the heating of he field system is within the thermal capability of the machine. Zero Sequence Impedance Connect all the three phases of the armature winding in series and apply a reduced voltage as shown in FIG.3. Drive the machine at rated speed with the field winding short-circuited. Then, X0=Z0= V/(3I) per phase. Where I is the current through each winding. Report: Positive Sequence Impedance X1= V/2I1 "= X2= V/2I2 "= X3= V/2I3 "= K=(1/3)(X1+X2+X3)= m = [ {(X1-K)2 + (X2-X3) 2}/2]1/2 Xd" = K-m Xq" = K+m Negative Sequence Impedance V= I= P= Z2 = V/(Ö3I)= X2 =PZ2/(VI)= Zero Sequence Impedance V= I= X0= Z0= V/(3I)= Result: Xd" = Xq" = X2= X0=


Experiment No-3 Objective: To determine sub transient direct axis reactance (xd) and sub transient quadrature axis reactance (xq) of an alternator

Theory & Concepts:

To understand the behavior of an alternator under transient conditions, the

armature and field resistance is assumed to be negligibly small. Thus, constant flux

linkage theorem can be applied.

As per this theorem, in purely inductive circuit, the total flux linkage cannot be

changed instantneously at the time of any disturbance.

Now, if all the three phases of unloaded alternator with normal excitation are

suddenly short circuited there will be short- circuit current flows in the armature.

As the resistance is assumed to be zero, this current will lag behind the voltage by

90o and the m.m.f. produced by this current will be along the d-axis. First

conclusion is that this current will be affected by d-axis parameters Xd , Xd′ and Xd″

only.

Further, there will be demagnetizing effect of this current, but as the flux linkage

with field cannot change the effect of demagnetizing armature m.m.f. must be

counterbalanced by a proportional increase in the field current.

This additional induced component of field current gives rise to greater excitation

under transient state and results in more short circuits as compared to the steady

state short circuit current.

If field poles are provided with damper bars, then at the instant of three phase


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short circuit, the demagnetizing armature m.m.f. induces currents in damper bars,

which, in turn, produces field in the same direction as the main field and hence at

this instant, the excitation further increases and gives rise to further increase in

short circuit armature current.

This is for a very short duration, normally 3 to 4 cycles and this period is known as

sub-transient period.

Since the field voltage is constant, there is no additional voltage to sustain these

increased excitations during sub transient or transient period.

Consequently the effect of increased field current decreases with a time constant

determined by the field and armature parameters and accordingly the short circuit

armature current also decays with the same time constant.

Fig 8.2: Symmetrical short circuit of an alternator

In the above figure a symmetrical wave from for armature short circuit current of

phase - A. The d.c. component is zero in this phase.

The reactances offered by the machine during sub transient period are known as


sub transient reactances. Along the direct axis, it is direct axis sub transient

reactance, X″d and along the quadrature axis, it is quadrature axis sub-transient

reactance, X″q. As these reactances are due to the fact that flux linkages in field

circuit during sudden disturbance remain constant, the sub transient reactances Xd″

and Xq″ can also be defined as below:

Direct axis subtransient reactance X″c,

The field structure is assumed to have damper bars on salient poles. The field

winding is initially unexcited and is short – circuited so that field flux- linkage is

zero. Armature currents now are suddenly applied in such time phase that the peak

of varying armature m.m.f. wave is in direct axis. As per constant flux linkage

theorem, since the flux linkage before this is zero. Hence, it remains zero just after

the application of armature m.m.f. wave and in order to maintain the flux linkages

zero, current are induced in damper bars, additional rotor circuit formed by pole-

body etc. and the field winding. The field of the varying armature m.m.f. is forced

to drive the flux through the leakage paths mainly in air as shown in Fig. 8.3.


Fig 8.3: Flux path for d-axis subtransient reactance (Xd″)

The armature flux linkage per ampere under these conditions is known as direct

axis sub transient inductance Ld″.

Fig 8.4: Flux path for q-axis subtransient reactance (X″q)

Quadrature axis subtransient reactance,Xq″


This also is defined in a manner similar to Xd″, but in this case, armature currents

are applied in such time phase that the peak of varying armature m.m.f. wave is

along the quadrature axis. The damper bars in the quadrature axis force the field of

the varying armature m.m.f. to follow the leakage path as shown Fig. 8.4.

As before, the flux linkage with q-axis damper bars must remain constant i.e. zero

before and after the sudden application of armature m.m.f. Under these

conditions, the armature flux linkages per ampere is known as q-axis sub transient

inductance Lq″ and Xq″=ωLq″.

To determine Xd″ and Xq″ in laboratory, the above mentioned conditions are

created there. Two phases of the three phase alternator are connected in series

and the combination is connected to a low voltage single phase supply. Field

winding is short circuited. The rotor is rotated and brought along the d-axis once.

Xd″ can be calculated from the armature current and voltage per phase of armature

in this position. Next, rotor is brought along the q-axis position and Xq″ is

determined.

Procedure:

(a) Open Circuit Test

1. Connect the circuit as per circuit Diagram.


2. Ensure that the external resistance in the field circuit of DC motor acting as a

prime mover for alternator is minimum and the external resistance in the field

circuit of alternator is maximum.

3. Switch on DC supply to DC motor and the field of alternator.

4. Start the DC motor with the help of stator. The starter arm should be moved

slowly, till the speed of the motor builds up and finally all the resistance steps are

cut out and the starter arm is held in on position by the magnet of no volt release.

6. Adjust the speed of the DC motor to rated speed of the alternator by varying the

external resistance in the field circuit of the motor.

7. Record the field current of the alternator and its open circuit voltage per phase.

8. Increase the field current of alternator in steps by decreasing the resistance and

record the field current and open circuit voltage of alternator for various values of

field current.

9. Field current of alternator is increase till the open circuit voltage of the

alternator is 25 to 30 percent higher than the rated voltage of the alternator.

10. Decrease the field current of alternator to minimum by inserting the rheostat

fully in the field circuit.


(b) Short Circuit Test

1. With the DC motor running at rated speed and with minimum field current of

alternator close the switch, thus short-circuiting the stator winding of alternator.

2. Record the field current of alternator and the short circuit current.

3. Increases the field current of alternator in steps till the rated full load short

circuit current. Record the reading of armature in both the circuit at every step. 4

to 5 observations are sufficient as short circuit characteristics is a straight line.

4. Decrease the field current of alternator to minimum and also decrease the speed

of DC motor by field rheostat of the motor.

5. Switch off the DC supply motor as well as to alternator field.

(c) Slip Test

1. Connect the circuit of alternator as shown in Fig ‘D’ keeping the connections of

the DC motor same.

2. Ensure that the resistance in the field circuit of DC motor is maximum.

3. Switch on the DC supply to the motor.


4. Repeat steps 4 described is (a).

5. Adjust the speed of the DC motor slightly less than the synchronous speed of the

alternator by varying the resistance in the field circuit of the motor. Slip should be

extremely low, preferably less that 4 percent.

6. Ensure that the setting of 3 phase Variac is at zero position.

7. Switch on 3 phase AC supply to the stator winding of alternator.

8. Ensure that the direction of rotation of alternator, when run by the DC motor

and when run as a 3 phase induction motor at reduced voltage (alternator provided

with damper winding can be run as 3 phase induction motor) is the same.

9. Adjust the voltage applied to the stator winding till the current in the stator

winding is approximately full load rated value.

10. Under these conditions the current in the stator winding the applied voltage to

the stator winding and the induced voltage in the open field circuit will fluctuate

from minimum values to maximum values which may be recorded by the meters

included in the circuit. For better results, oscillogram may be take of stator current

applied voltage and induced voltage in the field circuit.


11. Reduce the applied voltage to the stator winding of alternator and switch off 3

phase AC supply.

12. Decrease the speed of DC motor and switch off DC supply .

Result: We have performed the experiment and determine negative and zero

sequence reactances of an alternator

Experiment No-4 Objective: To study the IDMT over current relay and determine the time current

characteristics.

Apparatus: 1. Voltmeter (0-300 V) Digital 2. Ammeter (0-10 A) Digital 3. Loading C.T. 4. Auto Transformer 0-270V 5. Indicating Light 6. I.D.M.T. Relay Type CDG


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7. Timer with Start & Stop facility 8. Push Button for Timer START & STOP 9. Rotary Switch 10. DP Switch 11. Insulating terminals

Theory:

There are several over current protection such as fuse, thermal relay & IDMT Relay. IDMT (Inverse Definite Minimum Time) Relay is a high accuracy over current relay. If we does not want to flow the current in lines more than 1 Amp, we will set the tripping current in our relay 1 Amp. As the current will become 1.10 or 1.20, the relay disc will start forward and trip the breaker after certain time. It is widely used to prevent over current on transmission lines, power transformers etc, because the error & tripping time of the relay is tolerable by the lines and transformer. As the requirement of system is that the faulted line should be open instantaneously. If the faulted line breaker fails to open the faulted line, the next supply breaker have to be open to for making dead the faulty line. The next breaker may be at higher voltage line or the same voltage. The next breaker should open only after the first breaker failure. So we will allow approx 0.4 sec time to operate first breaker. If first breaker does not become open within 0.4 sec than it will be assume failure and the next breaker will become functional. These time and current distinguish is made by IDMT relay. Circuit Diagram:

Procedure:


Study the operating current & de-operating current of disc. (i) Keep the current source at minimum. (ii) The amp adj / relay test rotary switch is kept at AMP ADJ. (iii) Switch ON the test set. (iv) Increase the current source slowly and pay attention at disc of relay. (v) At certain current, it just moves in forward direction, this current is operating current and note the current. (vi) Now decrease the current through current source and pay hard attention at disc. (vii) The disc will stop at certain current and moves in reverse direction just after reducing the current. This current is de-operating current and note its value.

Result: We have draw the characteristics of IDMT relay after performing the test.

Objective and concept: To study ferranty effect and voltage distribution in H.V.

long transmission line using transmission line model.

Apparatus Used:

Transmission line model is consisting of four actions of transmission on line

operatable at 220V with current rating at 2A connected in pi network.

A continues variable power supply with two Digital voltmeter and two digital

ammeter mounted on front panel with Resistive, Inductive, Capacitive load fitted in

m.s. sheet complete with patch chords for interconnection. Additionally one LPF

Wattmeter is required if A.B.C.D. parameter with phase angle is to be calculated,

for which the calculation are given in our manual.

Theory:


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Transmission line model consists of four sections and each section represents 50

km long 400 KV transmission line. Parameters of 50 km long 400 KV Transmission

line are taken as :-

Series Inductance = 80 mH

Series Resistance = 2 ohm (In addition to resistance of inductance coil)

Shunt Capacitance = 0.47 microF

Leakage resistance or Shunt Conductance = 470 kohm For actual 400 KV

transmission lines range of parameter is :- l = Series Inductance = 1.0 to 2.0 mH/Km

r = Series Resistance = 0.5 to 1.5 ohm /Km

c = Shunt Capacitance = 0.008 to 0.010 microF/Km

g = Leakage resistance (Shunt Conductance) = 3 x 10–8 to 5 x 10–8 mho/Km

A long transmission line draws a substantial quantity of charging current. If such a

line is open circuited for a very lightly loaded at the receiving end, the voltage at

the receiving end may become higher then the voltage at the sending end.

This is known as ‘FERRANTI EFFECT’ and is due to the voltage drop across the line

inductance (due to the charging current) being in phase the sending end voltage.

The both capacitance and inductance are necessary to produce this phenomenon.

The capacitance and charging current is negligible in short line but significant in

medium length lines and appreciable in long lines. Therefore, phenomenon occures

in medium and long lines. In the phaser diagram, Ferranti effect is illustrated. The

line may be represented by a nominal pi circuit so that half of the total line

capacitance is assumed to be concentrated at the receiving end. OM represents the

receiving end voltage. OC represents the current drawn by the capacitance

assumed to be consetrated at the receiving end. MN is the resistance drop and NP

is inductive reactance drop. OP is the sending end voltage under no load condition

and is less than receiving end voltage.

Circuit Diagram:


Procedure:

(i) Apply the voltage (200 V max.) to the sending end and connect power factor

meter. Also connect 1 ammeter and voltmeter to each end (receiving and sending).

(ii) Connect the load comprising of R, L and C at the receiving end and note down

the value of receiving end voltage.

(iii) Now remove the load from the receiving end and note down the voltage on

receiving end. This voltage at the receiving end is quite large as compared to

sending end voltage.

Result:

We have performed ferranty effect and voltage distribution in H.V. long

transmission line using transmission line model.

Experiment No-1 · The unsaturated values of Xd and Xq for a 3-phase synchronous machine may be...

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